The invention relates to methods and apparatus for determining and recording three-dimensional ("3-D") topographies representing properties of objects.
Various systems exist that can be used to record 3-D images or topographies of surface properties of complex objects such as external human anatomic features (e.g., limbs or residual limbs), human organs (e.g., heart or brain), and skeletal structures, automobile body components, and industrial molds. For example, imaging systems such as computerized axial tomography ("CAT"), magnetic resonance ("MR"), or ultrasound imaging systems can be used to generate 3-D images of internal human structures based on X-ray, MR, or ultrasound images of "slices" of the structure. In these imaging systems, the apparatus is relatively large, and the patient lies inside the apparatus. The apparatus serves, in part, to precisely position the sensing elements. A set of two-dimensional cross-sections or "slices" are generated and the spacing from slice to slice is precisely controlled. The edges of each slice are detected, and a 3-D image is created from these edges. See, e.g., Nelson et al., "Visualization of 3D Ultrasound Data," IEEE Computer Graphics and Applications, 13:50-57 (1993).
Electro-mechanical systems use an electro-mechanical probe ("digitizer") to generate a digital representation of the precise locations of a multitude of points on the surface of an object, e.g., a plaster mold of a residual limb, along a prescribed, ordered path. Such systems are used, for example, in the field of residual limb shape measurement, and include the DIGITSHAPE.TM. (Shape Products, Ltd., W. Sussex, England, UK), and the MIND.RTM. digitizer (Model and Instrument Development, Inc.; Seattle, Wash., USA). In these systems, the object is rotated at a fixed rate, and the digitizer tip is moved along the axis of the object, also at a known rate; thus, the tip traces a precise, known helical path along the object. At each point along the path, the radial distance from the axis to the surface of the object is recorded. A 3-D image is then generated from the numerous points along the helical path. In these systems, the digitizer is positioned by a mechanism.
Digitizers which are positioned by hand have also been used to record shapes of objects. For example, the Otto Bock Shape System (Otto Bock Orthopaedic Industry of Canada, Ltd.; Winnipeg, Manitoba) uses a hand-held digitizer to record torso shape in the manufacture of custom-fit seating. The operator moves the digitizer along the surface of a mold of the torso. The operator enters a surface position sample by pressing a mouse button at operator-determined intervals. A single data point is recorded when the operator presses the mouse button, and data are recorded only when the operator presses the button. When sampling is completed, the torso shape is interpolated from these data points. Lemaire et al., J. Rehabil. Res. & Dev., 33:311-320 (1996) describes this method as relatively inaccurate.
Hand-held digitizers also have been used for data entry in other imaging systems. For example, Horstmann et al., Comput. Med. Imag. Graphics, 18:229-233 (1994), describes an ultrasonic system for the 3-D localization of small, hand-held surgical tools used during brain surgery. The position of the tool is sent to a visualization unit and overlaid in real-time on 3-D images, e.g., CT and magnetic resonance images, of the patient's brain scanned before the operation. The main component of this system is an ultrasound transducer that indicates the precise location of a pointing device that can be attached to surgical tools.
Systems for stereoscopic image-guidance of a probe during neurosurgery are also known (see, e.g., Peters et al., Comput. Med. Imag. Graphics, 18:289-299 (1994), and Peters et al., "Nuclear Science Symposium & Medical Imaging Conference," 1993 IEEE Conference Record, 3:1805-1809 (Klaisner et al., eds., IEEE, New York, N.Y., 1993). These systems use a probe connected to a base by an articulated arm whose coordinates are constantly relayed to a computer to allow the probe to be visualized stereoscopically in 3-D images, e.g., anatomical and vascular, of a patient undergoing surgery.
Other 3-D imaging systems use a beam of light, e.g., a laser, in a regular, geometric pattern to project light with observable non-uniformities onto an object's surface. The systems detect the non-uniformities from a plurality of viewpoints, and then determine the position of the non-uniformities, e.g., by triangulation. The set of non-uniformity positions is used as a set of points to create the 3-D image of the surface of the object. Examples of such "structured" light systems include laser-triangulation systems (e.g., used in industrial measurement systems), and optical probes (see, e.g., Massen et al., U.S. Pat. No. 5,372,502 and Kenet et al., U.S. Pat. No. 5,016,173). Other light-based systems use phase-shift and Moire interference patterns. Light-based systems are restricted to the measurement of visible quantities such as surface features and color.
In other imaging systems, colors are used to represent the difference in shape between two objects. For example, Boone et al. J. Rehab. Res., 31:42-49 (1994), describes the use of different colors to represent modifications to the contour of a 3-D image on a computer screen of a prosthetic socket to provide supporting or relieving forces. Borchers et al., J. Prosthetics Orthotics, 7:29-34 (Winter 1995), describes the use of different colors, generated after a sampling process is completed, to represent the shape differences between a foot and a shoe last.